The present invention relates generally to variable-crown rolls, and more particularly, to variable-crown rolls including a fixed, central axle and a roll mantle rotatably mounted on the axle defining a space therebetween in which apparatus for adjusting the nip profile formed by the roll with a counter-roll can be adjusted. Such nip profile adjustment apparatus can include hydrodynamic glide-shoes mounted on pressure fluid controlled loading-pistons or one or more pressure-fluid chambers occupying the space between the central axle and the inner surface of the roll mantle. The glide-shoe piston devices or the like are arranged to act substantially in the plane of the nip or symmetrically with respect to the nip. The roll mantle is typically rotatably journaled at regions proximate to both of its ends by means of bearings while the fixed central axle is typically supported at points axially outside of the points at which the roll mantle is journaled on the central fixed shaft.
Variable-crown rolls of the type described above are commonly used in paper machines to form dewatering press nips, smoothing nips, calendering nips and the like with counter-rolls. For such purposes, it is important that the distribution of the linear load, i.e., the profile of the nip in the axial direction of the rolls, is either constant or adjustable as desired, such, for example, so as to control the transverse moisture profile and/or thickness profile or caliper of the web. There are many examples in the prior art of variable or adjustable-crown rolls which are designed in an attempt to provide adjustability for the distribution of the linear load in a nip formed by such rolls and corresponding counter-rolls.
Conventional variable-crown rolls used in paper machines generally comprise either a solid or tubular, fixed central roll axle or shaft and a roll mantle rotatably mounted around the fixed axle. Hydraulic pressure-actuated glide-shoe arrangements and/or one or a series of pressure-fluid chambers are arranged within the space between the fixed axle and the roll mantle for aligning or adjusting the axial profile of the mantle at the nip. Generally, the nips formed by such variable-crown rolls, such as press nips or calendering nips, are loaded by means of forces applied to the axle journals of the variable-crown roll and the associated counter-roll.
The counter-roll forming the nip with the variable-crown roll generally is mounted on rotating axle journals which are permanently fixed to the mantle of the counter-roll. On the other hand, a variable-crown roll is journaled on the fixed, central axle, which is generally quite massive. The distance between the opposite journal bearings of the counter-roll usually differs from the corresponding distance between the journal bearings of the variable-crown roll, the latter usually being substantially smaller than the former, thereby resulting in certain drawbacks.
For example, the difference between the bearing distances of the variable-crown roll and its counter-roll can result in the shapes of the crowns of the variable-crown roll and its counter-roll differing from each other. Various attempts have been made to resolve this problem. For example the rolls can be crowned. However, this technique will result in a uniform nip profile at only one particular linear load. Heating devices have been used in calender rolls to heat the roll mantle at portions at which higher linear loads are desired. However, such techniques also have their own limitations and drawbacks. Attempts have also been made to obtain a uniform nip profile by designing the variable-crown roll to have an extended bearing distance. However, this technique only results in creating new problems in connection with the frame construction of the paper machine. It has also been suggested to use an articulated bearing and a rolling bearing, situated one over the other, in the journaling of a variable-crown roll and its counter-roll to obtain a uniform nip profile. However, this technique requires a significant amount of space. Still other techniques for obtaining a uniform nip profile have been suggested, such as so-called self-acting counter-moment rolls.
It has also been suggested to provide a variable-crown roll with an extended mantle so that the bearing distances of the variable crown roll and counter-roll are substantially equal. In such arrangements, the crown line of the variable-crown roll will substantially correspond to the crown line of the counter-roll whereby the caliper profile of the nip formed by the variable-crown roll and the counter-roll is substantially uniform. Extending the mantle of the variable-crown roll, however, results in numerous drawbacks.
For example, it is difficult to service the end regions of the roll mantle. The major part of the range of adjustment of the hydraulic loading elements, e.g., the hydrodynamic glide-shoe loading-piston devices, must often be used for correcting the deviation of the crown at the end regions of the roll. For this reason, the effective adjustment range available for correcting deviations in the mid-region of the roll is not as large as would be desirable.
Nip-forming roll couples which include a variable-crown roll are also known which do not require separate loading arms for applying nip-loading forces. In such arrangements, the length of the stroke of the glide-shoe loading-pistons is designed to be relatively large so as to be sufficient to produce the nip opening and closing movements of the roll mantle thereby eliminating the requirements for separate loading arms which normally act upon the ends of the roll axle. In such variable-crown rolls, the hydraulic loading elements which act upon the inner surface of the roll mantle also produce the nip pressure loading as well as crown variation and adjustability of the nip profile. However, these constructions also present certain drawbacks. For example, such variable-crown rolls are relatively complicated in construction, and, moreover, it is difficult to provide a stroke length for the loading pistons between the stationary central axle and the roll mantle which is sufficient to enable the nip to be opened and closed while at the same time obtaining adequate loading and a sufficient degree of adjustability for the nip profile. The end regions of the roll mantle have presented the greatest problems in this respect.
Regarding prior art relevant to the invention, reference is made to Finnish Patent No. 57,004, corresponding to U.S. Pat. No. 3,885,283, to Finnish Patent No. 67,923, corresponding to German Offenlegungsschrift No. 3,003,396, and to cancelled Finnish patent application No. 791,581, corresponding to published GB patent application No. 2,022,775.
A variable-crown roll is disclosed in Finnish Patent No. 57,004 in which the length of the stroke of the glide-shoe loading-pistons is designed to be relatively large so as to be sufficient for obtaining desired compression loading as well as the raising movement of the variable-crown roll mantle. The ends of the roll mantle are provided with guides for guiding the mantle in the direction of the nip plane so that no other devices are required for either compression loading or for opening and closing the nip.
A variable-crown roll construction is disclosed in Finnish Patent No. 67,923 in which power units are provided between the central axle and the ends of the roll mantle axially outwardly of the roll mantle bearings by means of which the roll mantle is subjected to forces for adjusting the loading profile of the mantle.
A variable-crown roll is disclosed in cancelled Finnish Application No. 791,581 which is, perhaps, most closely related to the present invention. In particular, a variable-crown roll is disclosed wherein pistons are arranged in cylinders formed in the fixed, central axle at positions which face the inner races of the roll mantle bearings. The pistons act exclusively for raising the roll mantle in order the open the nip and it is not possible for such pistons to produce forces which would load the nip.